Continuously variable transmissions (CVTs) utilizing both a planetary gear unit and a hydrostatic drive unit are well known. A variety of work machines use this type of transmission for industries such as agriculture, earth moving, construction, forestry, and mining. The hydrostatic drive unit allows the transmission to be operated in a hydro-mechanical mode in which a hydrostatic power flow is transmitted to the planetary gear unit from the hydrostatic drive unit and a mechanical power flow is transmitted to the planetary gear unit via a mechanical connection provided between the planetary gear unit and the engine. During operation, fluid displacement of the hydrostatic drive unit is varied to continuously change the output-to-input ratio of the transmission (i.e., the transmission ratio). This is accomplished by adjusting the angle of a swash plate of a variable displacement fluid pump or motor of the hydrostatic drive unit.
For current hydro-mechanical CVTs, shuttle shifting of the transmission requires the swash plate to be moved across its full range of travel at zero ground speed to shift from forward to reverse or vice versa. For example, FIG. 1 illustrates a graphical representation of the typical relationship between the transmission reciprocal ratio (i.e., the transmission output speed divided by the engine speed (or the transmission input speed)), denoted TRR, and the hydrostatic drive unit ratio (motor speed/pump speed), denoted HRR, of a conventional hydro-mechanical CVT having four selectable forward speed ranges and four selectable reverse speed ranges of operation: namely, forward speed range 1 or low (FR1); forward speed range 2 (FR2); forward speed range 3 (FR3); forward speed range 4 (FR4); reverse speed range 1 (RR1); reverse speed range 2 (RR2); reverse speed range 3 (RR3); and reverse speed range 4 (RR4). As is generally understood, the HRR is directly related to the swash plate angle of the pump of the hydrostatic drive unit. Accordingly, as the swash plate is moved, the transmission ratio, and, thus, the speed of the work vehicle is varied across a given speed range.
As shown in FIG. 1, for each of the speed ranges, the zero tilt position of the swash plate (e.g., indicated by the vertical TRR axis) lies between the maximum degrees of tilt in the opposite directions of movement of the swash plate. Thus, at the lowest HRR for the forward speed range FR1, the swash plate is typically at or near maximum tilt in the left hand or negative direction (which is also the zero speed ratio for the transmission for the forward direction). In addition, as shown in FIG. 1, to go from zero speed in the forward speed range FR1 to zero speed in the lowest speed range in the reverse direction (i.e., reverse speed range RR1), the swash plate must travel substantially its entire range of movement, as depicted by distance ROM. Thus, to perform a forward-to-reverse shuttle shift, not only must the forward and reverse directional clutches be swapped within the transmission, but the swash plate must also be moved the distance ROM. Such required movement of the swash plate significantly reduces the efficiency and smoothness while increasing the total time of shuttle shifting within the transmission.
Various systems have been proposed for addressing the issues associated with shuttle shifting within a conventional hydro-mechanical CVT. However, current systems still suffer from various drawbacks and fail to provide a complete solution for shuttle shifting.
Moreover, in addition to the difficulties associated with shuttle shifting, conventional hydro-mechanical CVTs also exhibit further issues when operating at low ground speeds. For instance, typical input-coupled systems start with large power generation and sacrifice system efficiency. Such hydraulic power regeneration not only reduces the system efficiency, but also increases the power through the mechanical branch of the transmission, which, in turn, imposes a significant limitation on the design of the mechanical gear train.
Accordingly, a system and method for operating a continuously variable transmission of a work vehicle in a manner that addresses one or more of the issues identified above is desired. In particular, a system and method for operating a continuously variable transmission of a work vehicle in a hydrostatic bypass mode that improves shuttle shifting performance and/or increases system efficiency at low ground speeds would be welcomed in the technology.